Such a quality factor tuning system is already known in the art, e.g., from the paper "A 10.7 MHz CMOS OTA-R-C bandpass filter with 68 dB dynamic range and on-chip automatic tuning", ISSCC February 1992, pp. 66-67, by M. Steyaert and J. Silva-Martinez. This paper more particularly concerns filters built with operational transconductance amplifiers (OTA's) and associated capacitances (C) which are commonly known as OTA-C filters. The use of this type of filter has proven to be particularly advantageous in the domain of integrated circuit analog filters. One of the advantages of such filters is derived from the controllability of the transconductances of the OTA's incorporated therein. Indeed, by tuning these transconductances the frequency response of the filter can be tuned. This tuning is primarily used to provide a very accurate frequency response in spite of inevitable implementation inaccuracies.
The present invention more particularly relates to a tuning system controlling such an OTA-C filter. It is to be noted that, although such a tuning system is not confined to the tuning of OTA-C filters but can be used in connection with any type of tunable filters, such OTA-C filters are the main application domain thereof. Therefore the tuning system will be further best described in connection with OTA-C filters.
Such tuning systems commonly operate according to the master-slave principle wherein at least one master filter included in a data processing path is tuned by a tuning signal, e.g. a tuning voltage for tuning a transconductance as mentioned above, generated by the tuning system which includes a replica of at least a section of this master filter. This replica is called a slave filter and corresponds to the above mentioned tunable filter. The tuning signal has to be so generated that the frequency reponse of the slave filter matches a desired response.
In the aforementioned document Steyaert et al it is proposed to determine this desired frequency response by tuning separately the quality factor of the total master filter and the time constant of the OTA-C integrators forming part of this filter. The former quality factor tuning is more particularly described in the section entitled "automatic tuning of the quality factor" and is the subject of the present invention. For the tuning of the time constant which is equivalent to a frequency tuning reference is made to the above-mentioned co-pending patent application of the same inventors as the present one, filed on even date herewith and entitled "Frequency tuning system for an OTA-C pair".
The quality factor of a filter is an important parameter as it determines for instance the bandwidth of the filter. In the known system use is made of the fact that the quality factor is correlated with the damping values of the circuit included in the filter. Hence, the quality factor is tuned by a tuning signal adjusting the resistances of the resistors included in the filter. For the generation of this tuning signal use is made of another well known fact in filter theory, namely that the shape of the transient response is determined by the quality factor. It is thus proposed to derive the tuning signal from the matching of the transient response of the tunable filter with a desired transient response.
The above described scheme indirectly tunes the filter's quality factor via its transient response. This scheme leads to a relatively long time delay for tuning the quality factor and furthermore requires relatively complex tuning circuitry as can be verified from the referenced document.